Morphological analysis of some cryptic species in the Acanthocyclops vernalis species complex from North America

Patterns of morphological variation and reproductive isolation were examined for several North American populations of copepods in the Acanthocyclops vernalis Fischer A., 1853 (Copepoda, Cyclopinae) species complex. The copepods were collected from six sites in Wisconsin, U.S.A. Morphological analysis of 120 adult females revealed that a character used previously to distinguish species in this group was unreliable because of phenotypic plasticity. Most of the morphological variance was due to environment (Laboratory vs. field) and to field site. Relatively little of the variation was due to measurement error or asymmetry. Multivariate ordination analysis produced poorly-defined clusters of individuals, suggesting that different biological species are difficult or impossible to distinguish using a set of easily-measurable morphological characters. In our study, morphological similarity was independent of geographic distance among sites, between 0.05 and 300 km. Isofemale lines within sites showed little or no reproductive isolation, but nearly complete isolation among sites. Reproductive isolation was also independent of morphology. These results suggest that the Acanthocyclops population at each site could be considered a distinct cryptic biological species. These copepods expressed morphological stasis – persistence of morphological uniformity despite reproductive isolation. Because of the effect of site and environment on morphology, we recommend using much larger collections (many sites), common garden experiments, and a multi-disciplinary approach (morphological, reproductive, chromosomal, and molecular) as the basis for future taxonomic research on putative copepod species.